1. Field of the Invention
This invention relates to a sensor utilizing localized plasmon resonance. This invention also relates to a sensor chip for use in the sensor, and a process for producing the sensor chip. This invention further relates to a fine structure body for use in the sensor of the type described above, and a process for producing the fine structure body.
2. Description of the Related Art
As disclosed in, for example, Patent Literature 1, there have heretofore been known sensors, in which a fine structure body comprising a dielectric material, a semiconductor, or the like, and fine metal particles secured in a layer-shaped form to a surface of the dielectric material, the semiconductor, or the like, is employed as a sensor chip, and with which a refractive index of a sample, or the like, is measured by the utilization of localized plasmon resonance. Basically, the sensors are provided with means for irradiation measuring light to the area of the fine metal particles of the sensor chip, and photo detecting means for detecting intensity of the measuring light coming from the fine metal particles secured in the layer-shaped form (i.e., the measuring light, which has passed through the fine metal particles, or the measuring light, which has been reflected from the fine metal particles).
With the sensors described above, when the measuring light is irradiated to the area of the fine metal particles secured in the layer-shaped form, the localized plasmon resonance occurs at a certain specific wavelength, and the scattering and the absorption of the measuring light are caused by the localized plasmon resonance to increase markedly. Therefore, in cases where the intensity of the measuring light coming from the fine metal particles secured in the layer-shaped form is detected, the markedly occurring attenuation of the detected intensity of the measuring light is capable of being observed, and the occurrence of the localized plasmon resonance is thereby capable of being confirmed.
In such cases, the light wavelength, at which the localized plasmon resonance occurs, and the extent of the scattering and the absorption of the measuring light depend upon the refractive index of the medium, which is present around the fine metal particles. Specifically, in cases where the refractive index of the medium, which is present around the fine metal particles, is large, a resonance peak wavelength shifts to the long wavelength side, and the scattering and the absorption of the measuring light increase. Therefore, in cases where the measuring light is irradiated to the area of the fine metal particles in a state in which a sample is located around the fine metal particles secured in the layer-shaped form, and the intensity of the measuring light coming from the area of the fine metal particles is detected, the refractive index of the sample, physical properties of the sample corresponding to the refractive index, and the like, are capable of being measured.
In such cases, white light may be employed as the measuring light, the light coming from the area of the fine metal particles may be detected spectrophotometrically, and the shift of the resonance peak wavelength described above may thereby be detected. Alternatively, monochromatic light may be employed as the measuring light, and the shift of the resonance peak wavelength described above, and a change in light intensity accompanying a change in scattering and absorption of the measuring light may thereby be detected.
Also, in order for the measuring light coming from the fine metal particles secured in the layer-shaped form to be detected, a photodetector may be located on the side with respect to the fine metal particles, which side is opposite to the measuring light irradiation side, and the light having passed through the fine metal particles may thereby be detected. Alternatively, the photodetector may be located on the side with respect to the fine metal particles, which side is identical with the measuring light irradiation side, and the light having been reflected from the fine metal particles may thereby be detected. In the latter cases, a base body, to which the fine metal particles are secured in the layer-shaped form, may be made from a material having light reflecting properties. In such cases, the measuring light having passed through the fine metal particles is reflected from the base body. Therefore, the measuring light, which has passed through the fine metal particles and has then been reflected from the base body, is capable of being detected together with the measuring light, which has been reflected from the fine metal particles.
Further, in cases where a sensing medium, which is capable of binding with a specific substance, is fixed to peripheral areas of the fine metal particles of the sensor chip, the refractive index at the peripheral areas of the fine metal particles alters in accordance with the occurrence of the binding of the sensing medium with the specific substance. Therefore, the measuring light may be irradiated to the area of the fine metal particles in the state in which the sensing medium described above has been fixed to the peripheral areas of the fine metal particles, and the intensity of the measuring light coming from the area of the fine metal particles may be detected. In this manner, the occurrence of the binding of the sensing medium with the specific substance is capable of being detected. The combination of the specific substance and the sensing medium may be, for example, the combination of an antigen and an antibody.
As the sensor chip for use in the sensor utilizing the localized plasmon resonance, for example, a sensor chip comprising a base body and a colloidal metal single layer film, which is formed at a surface area of the base body, has heretofore been known. The sensor chip comprising the base body and the colloidal metal single-layer film, which is formed at the surface area of the base body, is described in, for example, Patent Literature 1. Also, a fine structure body comprising layer-shaped anodic oxidation alumina, which has a plurality of fine holes formed in one surface, and fine metal particles, which are loaded in the fine holes of the anodic oxidation alumina, is applicable to the sensor described above. The aforesaid fine structure body is described in, for example, Non-Patent Literatures 1 and 2. Anodic oxidation alumina itself, which has a plurality of fine holes, is also described in, for example, Patent Literature 2 and Non-Patent Literature 3.
As described in, for example, Non-Patent Literature 3, it has also been known that a plurality of fine holes having diameters ranging from approximately several nanometers to approximately 300 nm are formed in a regular pattern in an anodic oxidation alumina film which is obtained from anodic oxidation processing performed with Al in a solution.
A marked feature of the anodic oxidation alumina acting as a porous material is that the anodic oxidation alumina has a honeycomb structure, in which the fine holes are formed in parallel at approximately equal intervals and extend in a direction approximately normal to the surface of the base plate. The anodic oxidation alumina also has the unique features in that the diameters of the fine holes, the intervals of the fine holes, and the depths of the fine holes are capable of being adjusted comparatively freely.
As described in, for example, Non-Patent Literature 4, it has also been known that an anodic oxidation alumina film may be formed on a base plate constituted of GaAs or InP, and fine holes may be formed in the GaAs base plate or the InP base plate with the anodic oxidation alumina film acting as a mask.
[Patent Literature 1]
                U.S. Patent Laid-Open No. 20020089617[Patent Literature 2]        Japanese Unexamined Patent Publication No. 11(1999)-200090[Non-Patent Literature 1]        Journal of Applied Physics, Vol. 49, No. 5, p. 2929, 1978[Non-Patent Literature 2]        Journal of Applied Physics, Vol. 51, No. 1, p. 754, 1980[Non-Patent Literature 3]        “High-Regularity Metal Nano-Hole Array Based on Anodized Alumina” by Hideki Masuda, Solid Physics, Vol. 31, No. 5, p. 493, 1996[Non-Patent Literature 4]        Masashi Nakano, et al., Jpn. J. Appl. Phys., Vol. 38, pp. 1052-1055, 1999        
A sensor chip comprising the layer-shaped base body, such as the anodic oxidation alumina, which has the plurality of the fine holes formed in one surface as described above, and the fine metal particles, which are loaded in the fine holes of the base body, is also capable of being used for the operation, in which the sensing medium, which is capable of binding with the specific substance, is fixed to peripheral areas of the fine metal particles, and the occurrence of the binding of the sensing medium with the specific substance is thereby detected.
However, in cases where the state of the binding of the sensing medium with the specific substance is to be detected by use of the conventional sensor chip constituted in the manner described above, the problems are encountered in that the change in sensor output signal arising due to the occurrence of the binding of the sensing medium with the specific substance (i.e., the shift of the resonance peak wavelength described above, or the change in light intensity accompanying a change in scattering and absorption of the measuring light) is weak, and a long period of time is required before the change in sensor output signal arising due to the occurrence of the binding of the sensing medium with the specific substance is found.